In the electrochemical sensor described in German Pat. No. 31 20 159, the danger exists that during the operation of the heating element, particularly if there is insufficient insulation between the heating element and the oxygen-ion-conductive solid electrolytes which, for example, can be made of yttrium-stabilized ZrO2 (YSZ ceramic), leakage currents will occur which electrically couple the sensor cell to the heating element. First of all, such an electrical coupling reduces the service life of the heater, since reduction effects occur in the active ceramics, and secondly, the measuring signals emitted by the sensor are increasingly and permanently invalidated. Given continuous occurrence, the leakage currents lead to a local blackening of the sensor. In addition, the thin heating lines of the resistance heating element can burn through due to the local heating. In the case of the known sensor, a further disadvantageous effect occurs because of the interspersing of interference signals from the heating element, operated with pulsed voltage, into the probe signal, whereby the measuring accuracy drops because of the reduced signal-to-interference ratio.
An object of the present invention is to prevent an electrical coupling from the solid-electrolyte sections of the sensor to the heating element during its operation. Furthermore, the intention is to construct a sensor according to the present invention in such a way that blackening no longer occurs during the check for leakage current. Moreover, a sensor of the present invention is to be constructed in a manner that the service life of the heating element is extended. In addition, a sensor according to the present invention should be able to deliver a stable measuring signal over its service life. A sensor of the present invention should also be constructed so that no interference signals from the heating element are interspersed into the measuring-active ceramics, and thus into the sensor signal. A further intention is that the sensor of the present invention be so designed that the accuracy of the measuring signal is improved.
In an electrochemical sensor, designed according to the present invention, for ascertaining gas concentrations in gases, particularly in exhaust gases of internal combustion engines, having an oxygen-ion-conductive solid electrolyte which is provided with electrode layers arranged at a distance from one another and with at least one resistance heating element that is separated from the solid electrolyte by an electrical insulating layer, at least one foil binder layer being provided between the electrical insulating layer(s) and the solid electrolyte, at least one electron-conductive intermediate layer is provided between the electron-side electrical insulating layer and the adjacent solid electrolyte.
In one preferred specific embodiment, the electrochemical sensor of the present invention has a thin electron-conductive metal layer at least above the resistance heating element. This metal layer can either be imprinted flat-spread as a platinum-containing paste at least over the hot region of the sensor, or else can be applied in the form of a platinum lattice structure at least over the hot region of the sensor. Alternatively, the platinum lattice structure or the imprinted layer made of platinum paste can also lie over the entire surface, i.e., over the hot regions and the leads of the resistance heating element.
The platinum lattice structure can have lattice bars running at right angles, i.e., parallel to the edges of the sensor, or else running diagonally at a specific angle.
In one specific embodiment, the electron-conductive intermediate layer, such as the platinum lattice or a platinum mesh, can lie directly over the electrical insulating layer. Alternatively, the electron-conductive intermediate layer, i.e., particularly the platinum lattice or the platinum mesh, can replace or so modify one of the foil binder layers in the sensor that this/these foil binder layer(s) have sufficient electron conductivity. At the same time, the thermal conductivity of the construction counteracts local overheating of the heater.
To reduce or screen off the interference signals coupled in from the resistance heating element, the electron-conductive intermediate layer or intermediate layers, such as the platinum lattice, can be electrically connected to a defined potential, in particular to earth (ground) potential in the sensor.